Low-density polyethylene (LDPE) produced by a high-pressure radical polymerization process is branched polyolefin having a tree-like structure with non-linear side branches. The branched polyethylene exhibits enhanced melt-flow characteristics and melt strength, and thus, is advantageous in melt-shaping characteristics. But, its solid polymer has a low mechanical strength and its molten polymer has a reduced drawability.
In contrast, high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), prepared by polymerization using a Ziegler catalyst or a metallocene catalyst, have high mechanical strength and melt drawability. Therefore, HDPE and LLDPE are widely used in a field where high mechanical strength and melt drawability are required. However, HDPE and LLDPE have poor processability.
To improve processability of HDPE and LLDPE, various processes for producing polyethylene have been proposed. The proposed processes include, for example, (i) a process wherein polymerization is conducted in a multi-stage using a conventional Ziegler catalyst whereby a polymer having a broad molecular weight distribution is obtained (see Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. H2-53811, JP-A H2-132109 and JP-A H10-182742; (ii) a process wherein ethylene is polymerized using a conventional chromium catalyst to give polyethylene having long side branches; (iii) a process wherein ethylene is polymerized using a specific metallocene catalyst to give polyethylene having long side branches (see U.S. Pat. No. 5,272,236); (iv) a process wherein ethylene is copolymerized with a macromonomer using a specific metallocene catalyst with an enhanced degree of copolymerization to give polyethylene having long side branches (see WO 94/07930); and (v) a process wherein an ethylene-macromonomer copolymer is blended with linear polyethylene (U.S. Pat. No. 6,114,457).
Processability of polymers prepared by the above-proposed processes are not improved to a sufficient extent. The polymers prepared by the above-mentioned processes (i) and (ii) have a broad molecular weight distribution and hence have reduced mechanical strengths. The polymers prepared by the above-mentioned processes (iv) and (v) have an irregular particulate form and therefore a slurry polymerization process cannot be adopted for the production of these polymers.